In order to reduce the landing distance of a jet engine powered aircraft, as well as to increase the margin of safety when the aircraft is landing on a wet or icy runway, aircraft jet engines are provided with thrust reversers in order to provide a braking thrust for the aircraft. Typically, such thrust reversers are formed by thrust reverser "doors" which are capable of pivoting between two positions about an axis which is transverse and substantially diametrical with respect to the jet of the engine.
The first position finds the doors in a stowed position, out of the direct path of the exhaust blast of the engine. In this position, the doors form the exhaust nozzle of the gas turbine engine so that the thrust of the engine is directly rearward, thereby producing the forward thrust of the aircraft. In the second position, the doors are pivoted about the pivot axis to a transverse, blast deflecting or deployed position, to intercept and redirect the jet blast and produce the braking thrust for the aircraft when needed.
The jet engines which utilize such thrust reversers are typically of the double jet type of gas turbine engines. Such engines comprise a central generator which emits a jet of hot gas, and an annular by-pass conduit surrounding the central generator, and through which passes a jet of relatively cold gas. In practice, the central or hot jet emits gas flow at a high temperature on the order of 500.degree. C. to 600.degree. C., while the by-pass flow is at a temperature considered relatively cold, i.e. on the order of 100.degree. C.
When a thrust reverser is used with such double flow engines, the thrust reverser re-directs in a forward direction the hot and cold flows, in order to produce the braking thrust. Since the hot, central flow impinges on the thrust reverser doors, they must be made of heat resistant material. In general, the material used for the thrust reverser doors is either steel or a titanium alloy. However, steel is a relatively heavy metal, and while titanium alloys are much lighter, they are considerably more expensive. Aluminum is a very desirable material for the thrust reverser doors, but because of the high temperature of the hot flow, aluminum cannot be used.
In the past, attempts to overcome the drawbacks of conventional double flow engines have been made, for example in U.S. Pat. Nos. 4,362,015 and 4,581,890. For example, it has been proposed to provide an obstruction in the nature of flaps for obstructing a portion of the discharge outlet area of the exit end of the by-pass conduit when the thrust reverser is deployed. According to U.S. Pat. No. 4,362,015, the flaps restrict the flow of cold gas in by-pass conduit. In U.S. Pat. No. 4,581,890, the flaps are said to briefly reduce the efficiency of the flow mixing and causing the cold by-pass flow to contact the structure of the reverser, which was thereby maintained at a lower temperature. More precisely, this prior system increased (in thrust-reversing mode) the value of the ratio of the total pressure of the by-pass flow (i.e. the cold flow) to the total pressure of the hot flow, so that only the cold flow impinges on the reverser doors, thereby enabling the use of aluminum alloy doors.
However, it has been found that when such systems are applied to turbine engines that significant drawbacks arise. In essence, tests have shown that the engine rating must be kept reduced in the reverse mode in order to avoid surging the engine and raising the skin temperature of the thrust reverser doors. This is particularly true when the central nozzle of the gas generator is of the mixer type. The limitations found in the prior art systems are primarily due to the means used to control the value of the ratio of the fan pressure to the core pressure P.sub.T(fan) /P.sub.T(core). The prior art systems sought to increase this ratio by directly changing the flow characteristics of the fan and increase the fan pressure by mechanically restricting the fan flow by-pass area in the reverse mode, while only indirectly affecting the core flow.
As described in U.S. Pat. Nos. 4,362,015 and 4,581,890, these systems directly act on the total pressure of the fan flow so as to increase it, thereby increasing the ratio. This is done by using flaps in the cold flow to increase the pressure of the cold flow. However, the prior systems have no significant direct effect, and only insignificant indirect effect, on the value of the total pressure of the core flow. Ordinarily, the ratio would be on the order of 0.9-1.0, and through the use of the flap techniques disclosed in these prior patents, the ratio was increased to a value at most on the order of 1.2. Experience has shown that in some cases it has been necessary to install more than two, for example 3 or 4, flaps in the fan stream to adequately increase the pressure ratio of P.sub.T(fan) /P.sub.T(core) in reverse. As a result, the value of the total pressure ratio (fan/core) is increased in reverse, but mainly by the increase of the P.sub.T(fan). However, this is not desirable for the engine, as it increases the risk of surging of the engine. It is significant that in accomplishing the increase in the ratio, only the by-pass flow was acted upon, and this by the use of two or more flaps obturating the fan stream at the outlet end of the bypass conduit.
Accordingly, a primary object of the present invention is to overcome the drawbacks of the prior art thrust reverser systems.
Another object of this invention is to provide a thrust reverser which can use lightweight, inexpensive alloys such as aluminum, or composite materials, for the thrust reverser doors.
A further object of the invention is to provide an improved thrust reverser which operates with a significant increase in the ratio of the pressure of the fan flow to the pressure of the core flow when the thrust reverser is deployed.
Yet another object of the invention is to provide an improved thrust reverser system which increases the ratio of the total fan flow pressure to the total core flow pressure by directly acting upon the core flow.
Still another object of the invention is to provide an improved thrust reverser system which increases the ratio of the total fan flow pressure to the total core flow pressure by increasing the area of the hot gas nozzle while decreasing the area of the fan duct.
Still a further object of the invention is to provide an improved double flow jet engine which enables the use of a thrust reverser made of lightweight materials, by providing a nozzle which enables a mixing of the hot and cold flows with simultaneous decreasing of the total pressure of the hot flow and increasing the total pressure of the cold flow.
Another object of this invention is to provide a process for improving the efficiency of a double flow turbine engine by adjusting the mixing of the cold and hot flows, thereby simultaneously increasing the total pressure of the cold flow and decreasing the total pressure of the hot flow, by acting directly and primarily on the hot flow stream.
These and other objects and advantages of the invention will become apparent from a detailed consideration of the following description and claims, when taken together with the accompanying drawings.